1. Field of the Invention
This invention pertains generally to antennas, and more particularly to compact transmission line antennas.
2. Description of Related Art
Portable devices have become one of the necessary appliances for our daily lives. To conveniently carry these portable devices such as cell phones, media players and laptops, they are designed to be compact and lightweight, without sacrificing performance or functionality. The challenge to implement such small devices is to mount all the necessary circuits onto a small highly integrated transceiver unit. Among all the components, the antenna is one of the most challenging to scale down in size because the size of conventional antennas depends on operating frequency which is usually in the MHz or low GHz range. The traditional half-wavelength antenna cannot be incorporated in the space-limited RF front-end modules. Therefore, many researchers are investigating different methods to realize small antennas.
It has been shown that a reactive load attached to an antenna can lower the operating frequency and thus reduce the size of the antenna. Internal antennas including the Planar Inverted-F Antenna (PIFA) and chip antennas have also attracted attention because of their ease of integration with RF modules. The PIFA size can be reduced by several methods such as using a capacitive load or increasing the current flow path. In addition, the use of monopoles with circular disks loaded at the end, or the helix dipole antenna with spiral arm, have been shown to enhance impedance bandwidth within a compact size.
Recently, metamaterial based transmission lines have been developed and have been shown to exhibit unique features of anti-parallel phase and group velocities with a zero propagation constant at a given frequency for the fundamental operating mode. These metamaterials have been used to realize novel planar antennas, such as those exhibiting zeroeth-order resonant mode, which is characterized as having an infinite wavelength. In this case, the transmission line length is independent of the resonant phenomena, thus enabling physical size reduction. Zeroeth order resonators are described by inventors Tatsuo Itoh, Atsushi Sanada and Christophe Caloz in U.S. patent application Ser. No. 11/092,143 filed on Mar. 28, 2005, and published on Mar. 30, 2006 as U.S. patent application publication No. U.S. 2006/0066422 A1, both of which are incorporated herein by reference in their entirety.
In addition, the use of an L-C loaded transmission line has been used to create a λ/2 field distribution, where λ is the free space propagating wavelength, over a shorter line length to realize a smaller patch antenna and slot antenna compared to conventional antennas. Another method to reduce antenna size relies on the possibility of filling a cavity with a pair of double-negative, double-positive and/or single negative material blocks to synthesize the sub-wavelength cavity resonator.
None of these attempts, however, have been entirely successful at reducing antenna size without unduly sacrificing gain and other positive antenna characteristics.
Accordingly, a need exists for an antenna apparatus that can be implemented in a compact size while providing a high level of gain. These needs and others are met within the present invention, which overcomes the deficiencies of previously developed antenna structures.